The present invention relates generally to a solution polymerization method for high molecular weight polyimides. More particularly, the invention relates to a method for preparation of a high molecular weight polyetherimide by polymerizing diamine monomers and dianhydride monomers in a dual solvent system.
Polymerization of polyetherimides by melt polymerization or conventional solution polymerization processes using aromatic bis(ether anhydride) monomers (hereinafter "dianhydride") and organic diamine monomers is known in the art. See for example, Howson, U.S. Pat. No. 4,650,850; and Klopfer, U.S. Pat. No. 4,565,858. Polyetherimide polymers represented by formula I can be made by effecting a polymerization reaction between said dianhydride and diamine monomer, ##STR1## where R-R.sup.5 are independently H or C.sub.(1-4) alkyl radicals, R.sup.6 is a divalent arylene radical selected from the class consisting of C.sub.(6-14) hydrocarbon radicals, substituted C.sub.(6-14) hydrocarbon radicals and a divalent radical having the formula EQU --R.sup.7 QR.sup.8 -- (II)
where R.sup.7 and R.sup.8 are the same or different C.sub.(6-14) divalent aromatic hydrocarbon radicals or substituted C.sub.(6-14) divalent aromatic hydrocarbon radicals, Q is a member selected from ##STR2## and x is a whole number equal to 1 to 5 inclusive.
As described by Howson in U.S. Pat. No. 4,650,850 and in Heath et al., U.S. Pat. No. 3,847,867, polyetherimides comprising chemically combined etherimide units are useful for the production of injection molded high performance thermoplastics. The utility of these polymers in high heat applications is due, at least in part, to their relatively high glass transition temperature (Tg). Consequently, polyetherimide polymers are also useful in electronic applications. In addition, polyetherimides have been found to be useful in semiconductor fabrication due to its recently discovered adhesive properties.
In general, the higher the molecular weight, the better performance of a polymer, especially in terms of its mechanical properties. However, an ultimate upper limit is imposed on the molecular weight of polyetherimide polymers due to limitations in known melt and solution polymerization processes.
Melt polymerization processes are conducted at temperatures which exceed the melt temperature of the monomers being polymerized in order to promote polymerization by causing the nitrogen containing ring of the diamine monomer to close. This is referred to as imidization. However, due to the relatively high melt temperature of the polyetherimide polymer being produced, the melt viscosity during a typical melt polymerization process frequently becomes too high for extensive polymerization to occur.
The melt viscosity of a polymer is an indication of its ease of processability in the melt. A polymer with a high melt viscosity can become difficult to mix thus hindering further polymerization of the monomers. The following universal relationship between the molecular weight and the melt viscosity for a polyetherimide polymer is expected: EQU Z=kM.sup.3.4 (IV)
where Z is the melt viscosity, M is the molecular weight and k is a constant.
The melt viscosity of the polyetherimide polymer therefore increases by a power of 3.4 as the molecular weight increases during polymerization. For example, a 10% increase in molecular weight results in a 38% increase in the melt viscosity. Thus, limiting the molecular weight of the polymer is extremely important for trouble-free, safe operation of melt polymerization processes and for the production for a viable polyetherimide product.
Solution polymerization methods also have significant shortcomings when used to prepare high molecular weight polyetherimides. Solution polymerization processes must use a solvent with a sufficiently high boiling point below which imidization of the monomers occurs. Frequently, the solvents with elevated boiling points do not exhibit good solubility characteristics with the polymerized product. As the molecular weight of a polymer increases through polymerization, the intrinsic viscosity of the polymer also increases according to the following Mark-Houwink equation: EQU IV=(K)M.sup..alpha. (V)
wherein IV is intrinsic viscosity, K is a constant, M is the molecular weight and .alpha. is constant for a given polymer.
The intrinsic viscosity of a polymer is the limiting value at infinite dilution of the ratio of the specific viscosity of the polymer solution to its concentration in moles per liter. Intrinsic viscosity (n.sub.1) is defined as: ##STR3## where n.sup.sp is the specific viscosity and c is the concentration in moles per liter.
Solution polymerization of high molecular weight polyetherimides in the conventional solvents having a sufficiently high boiling point of at least about 130.degree. C. such as orthodichlorobenzene (BP=180.5.degree. C.) fail because the ever increasing viscosity of the polymer solution makes it impossible to stir, thereby hindering uniform mixing. Thus, extensive polymerization is impossible.
Presently, high molecular weight polyetherimides are prepared from 4,4'-sulfonyl dianiline (SDAN) and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride commonly referred to as bisphenol A dianhydride (BPADA) and having the following formula: ##STR4## These polyetherimides cannot be synthesized conveniently by either melt or solution polymerization methods to achieve an intrinsic viscosity (IV) of greater that 0.55 deciliters per gram (&gt;0.55 dl./g.).
Therefore, since a higher molecular weight polyetherimide polymer is desired there is an outstanding need for an improved method to polymerize polyetherimides to achieve a higher molecular weight. Ideal polymerization conditions would provide for a system having a temperature at least as high as the temperature at which polymerization occurs while at the same time having solubility characteristics which maintain a homogeneous stirrable mixture of monomer and polymerized product throughout the course of the entire polymerization. In general there is no teaching in the prior art of a method for polymerizing a high molecular weight polyetherimide using such a system.